If there are sheets that you want in File A, but they are in File B,
try this:

Open both files.

Select all the sheets that are File B (hold down the Ctrl key while
selecting the tabs) then right click and select 'Move or copy'...select
File A in the To book: dropdown.

If you have Create a Copy checked off at the bottom, it will copy those
sheets you selected. If Create a Copy is NOT checked off at the bottom
of the dialog box, it will MOVE the sheets you selected to File A from
file B.

HTH